1. Field of the Invention
The invention relates to a method for producing alumina hydrates by precipitating aluminum salts in the presence of crystal nuclei of a specific size.
2. Description of the Prior Art
Known in the art are procedures for manufacturing boehmitic alumina or alpha-aluminum oxide monohydrates by neutralizing basic aluminate solutions, e.g., with sulfuric acid or acidic aluminum salt solutions. In this case, the alumina hydrates are generally precipitated at a pH value of between 4.5 and 7. However, the literature also describes procedures in which the alumina hydrates are precipitated at a pH of 7 to 10.
The use of crystal nuclei during the precipitation of alumina is also known. In the procedure described in DE 21 25 625-C2, a sodium aluminate solution is reacted with nitric acid in a first boiler, precipitated in a second boiler, wherein a partial flow is removed from the latter and routed to the first boiler. According to the procedure described in DE 21 25 625-C2, pore volumes of up to 0.33 cm3/g are obtainable.
U.S. Pat. No. 4,154,812 describes a procedure for the manufacture of alumina hydrates that involves 5 phases:    1. Formation of crystal nuclei from an aqueous aluminum salt solution by adding aluminum sulfate in water and setting a pH of 2 to 5.    2. Precipitation of the boehmitic alumina by setting a pH value of between 7 and 8 via the simultaneous addition of basic aluminate and acidic aluminum salt solutions.    3. Setting of a pH value of between 9.5 and 10.5 by adding a corresponding quantity of aluminate solution.    4. Repeated stirring of the alumina suspension.    5. Filtration of the suspension and washing of the filter cake.
The procedure according to U.S. Pat. No. 4,154,812 is characterized by the observance of defined pH values and temperatures (54 to 82° C.) during specific phases of the reaction.
The boehmitic alumina obtainable according to this procedure exhibit a pore volume (0-60 nm) of less than or equal to 0.82 ml/g after an activation (400° C. for 30 minutes). These products exhibit 0.02% w/w Na2O and 0.2% w/w sulfate relative to 72.5% w/w Al2O3 as the primary contaminants. U.S. Pat. No. 4,154,812 describes the porosity of the boehmitic alumina as being influenced by the pH value, concentration and temperature. At the arising pH value of 2 to 5, or 3 to 4, partial hydrolysis of the acidic aluminum salt produces boehmitic alumina in the form of crystals given a high dilution and a temperature of 140 to 170° F. These crystals act as nuclei during the subsequent precipitation. Obtained in this way are boehmitic silica that exhibit pore volumes of 0.8 to 0.9 ml/g.
U.S. Pat. No. 4,248,852 describes a procedure for manufacturing γ-alumina with a high pore volume. According to this publication, aluminum salts and aluminates are reacted in such a way that several reaction phases with respectively different pH values (pH-swing) must be traversed.